Zeolite is used as a catalyst, a catalyst carrier, an absorbent, or the like, and a zeolite laminated film formed on a porous substrate made of metal or ceramic is used for a gas separation membrane or a pervaporation membrane by making use of a molecular sieving effect of the zeolite. In view of these circumstances, zeolite membranes using various porous substrates and production methods of the same have been proposed. Such a zeolite formed into a thin membrane is used as a molecular separation membrane of a gas separation membrane, a pervaporation membrane, or the like by its molecular sieving effect.
There are many types of zeolites such as LTA, MFI, MOR, AFI, FER, FAU, DDR depending on crystal structures. Among them, DDR (Deca-Dodecasil 3R) is a crystal mainly containing silica, and its pore is formed by a polyhedron with an oxygen eight-membered ring, and a known diameter of the pore of the oxygen eight-membered ring is 4.4×3.6 angstroms (see W. M. Meier, D. H. Olson, Ch. Baerlocher, Atlas of Zeolite Structure Types, Elsevier (1996)).
A DDR type zeolite having such a structural feature is a zeolite having a relatively smaller pore diameter, and can be applied to a molecular sieve membrane for separating a gas of low molecular weight such as carbon dioxide (CO2), methane (CH4), ethane (C2H6) that are hard to separate by other zeolites such as MFI, MOR, AFI, FER, and FAU types.
There have been proposed only a few production methods for such DDR type zeolites. Some proposals have been made on powder synthesis, but no proposals have been made on the synthesis of the membrane. As one of the few proposals related to powder, for example, M. J. den Exter, J. C. Jansen, H. van Bekkum, Studies in Surface Science and Catalysis, Vol. 84, Ed. by J. Weitkamp et. al., Elsevier (1994), 1159–1166 discloses a method for producing a DDR type zeolite powder by hydrothermal synthesis using tetramethoxysilane, 1-adamantanamine, ethylenediamine, or the like, as raw materials.
However, the method for producing a DDR type zeolite powder requires hydrothermal synthesis using an autoclave at 160° C. for a long period of 25 days, and requires constant stirring of a raw material solution, thus is not a convenient method.
Further, this production method provides only a DDR type zeolite powder with a crystal size of 5 to 25 μm, and thus has a problem that it cannot produce a dense separation membrane having a sufficient thickness for a gas separation process in a petrochemical industry or the like.
The present invention (a first aspect of the present invention) is made in view of the problems of the related art, and has an object to provide a method for easily producing a DDR type zeolite membrane having a thickness that is acceptable in an industrial gas separation process or the like and in a short period of time.
No conventional zeolite membrane has a pore diameter that can separate independently a molecule of a relatively low molecular weight such as methane (CH4), ethane (C2H6), propane (C3H8), carbon dioxide (CO2), or the like contained in a natural gas.
Known as a refining method of a natural gas is a method using a separation membrane mainly containing an organic substance that selectively passes carbon dioxide (CO2) rather than methane (CH4). However, an organic membrane has low mechanical strength and low heat resistance, is plasticized by carbon dioxide, and is deteriorated by liquid hydrocarbon. Thus, there is no membrane suitable for membrane separation of a gas containing a molecule of a low molecular weight such as a natural gas.
The present invention (a second aspect of the present invention) is made in view of the above described problems, and has an object to provide a DDR type zeolite membrane having high mechanical strength and high heat resistance, capable of causing hardly such problems as plasticization by carbon dioxide and deterioration by liquid hydrocarbon, and separating one or more types of particular gas components from a mixed gas containing two or more types of particular gas components such as a natural gas; a gas separation method using a DDR type zeolite membrane; and a gas separation apparatus being comprised of a DDR type zeolite membrane.
Generally, a zeolite membrane formed into a thin membrane has low mechanical strength, and thus a zeolite membrane is formed on a porous substrate made of metal or ceramic to form a zeolite membrane composite to increase the mechanical strength. Besides the increase in the mechanical strength, such a zeolite membrane can form a thin zeolite membrane, and thus can increase a gas flux when the membrane is used as a gas separation membrane, compared to when used as a self-supported membrane of a zeolite alone.
For the zeolite membrane formed on the porous substrate, thermal expansion behaviors significantly differ between the porous substrate and the zeolite membrane, which may cause defects such as cracks in the zeolite membrane on the porous substrate when a structure-directing agent used in zeolite synthesis is removed by calcination.
Thus, Vroon et al. of University of Twente propose that the defects such as cracks can be effectively prevented by making the thickness of the zeolite membrane to be formed on an alumina substrate thinner (See Journal of Membrane Science, 144 (1998), pages 65–76). However, the probability in the occurrence of defects such as cracks due to thermal stress cannot be reduced sufficiently by simply making the zeolite membrane thinner.
There is disclosed in JP-A-9-202615 a zeolite membrane having a zeolite crystal on a porous substrate and a production method of the same. It describes that the occurrence of defects such as cracks due to thermal stress in the thermal treatment process of the zeolite membrane and the like can be reduced by employing such a constitution that a porous substrate possesses zeolite crystal in the inside thereof.
However, there is a problem that the zeolite membranes disclosed in JP-A-9-202615 and the like cannot always be applicable to all types of zeolites. For example, for an LTA type zeolite, defects such as cracks of a zeolite membrane formed on a porous substrate cannot be decreased. The above described advantage varies, depending upon materials of the porous substrate, and for example, when alumina is used for a porous substrate, and an MFI type zeolite is used for a zeolite membrane, the defects such as cracks or the like occur after the zeolite membrane is used a few times.
Incidentally, in the case of the zeolite membrane disclosed in JP-A-9-202615, a porous substrate of 0.5 mm or more in thickness is used since an amount of raw material solution being present in the porous substrate is important. If a zeolite membrane composite thus configured is too thick when used as a gas separation membrane, there is such a problem that pressure loss becomes large, thereby a sufficient gas flux can not be attained.
The present invention (a third aspect of the present invention) is made in view of the above problems, and has an object to provide a DDR type zeolite membrane composite that has high mechanical strength, can prevent occurrence of defects such as cracks due to thermal stress, and can secure a sufficient flux, when a DDR type zeolite membrane composite is used as a gas separation membrane, and a production method of the same.